How many kilocalories are needed to vaporize 18.0 of H2O?
What is the unit for 18.0; moles, milligrams, liters, etc.? Without it I don't see how it's possible to solve. After applying the appropriate unit given in the question try solving it again.
Hint:
1) 40.65 kilojoules/mol is the heat of vaporization for water.
2) 1cal equals 4.1868 joules.